Two Treatises of Government, John Locke

https://oll.libertyfund.org/titles/locke-the-two-treatises-of-civil-government-hollis-ed

The Founders were avid students of history and philosophy. One of the most widely read political philosopher of their time was John Locke. In his second treatise of government, John lays out a detailed argument about how we know our rights come from God.

The Records of the Federal Convention of 1787, Max Farrand

https://oll.libertyfund.org/titles/farrand-the-records-of-the-federal-convention-of-1787-vol-1

When studying the Constitution, it is critical that we understand the original intent so that we do not simply impose our own interpretation or desires on the meaning. To this end, the best way to understand this intent is to know what the Framers said about the clauses at the time of its adoption. Max Farrand pulls together the notes and letters of various delegates to the convention in this indexed and well-sourced compilation. It is a MUST HAVE for any true student of the Constitution.

The Naked Communist, W. Cleon Skousen

Cleon Skousen was a dedicated FBI employee who served under J. Edgar Hoover. After being alerted to the threat of Communism, he sought to learn as much as he could about the scourge. He discovered that the information about Communism could not be found in one place and he subsequently remedied this problem in this excellent book which has become the reference manual on the subject.